netdev - Re: [PATCH v2] rhashtable: make walk safe from softirq context

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Date:   Tue, 12 Feb 2019 10:43:39 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     johannes@...solutions.net
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org, j@...fi,
        tgraf@...g.ch, herbert@...dor.apana.org.au, johannes.berg@...el.com
Subject: Re: [PATCH v2] rhashtable: make walk safe from softirq context

From: Johannes Berg <johannes@...solutions.net>
Date: Wed,  6 Feb 2019 10:07:21 +0100

> From: Johannes Berg <johannes.berg@...el.com>
> 
> When an rhashtable walk is done from softirq context, we rightfully
> get a lockdep complaint saying that we could get a softirq in the
> middle of a rehash, and thus deadlock on &ht->lock. This happened
> e.g. in mac80211 as it does a walk in softirq context.
> 
> Fix this by using spin_lock_bh() wherever we use the &ht->lock.
> 
> Initially, I thought it would be sufficient to do this only in the
> rehash (rhashtable_rehash_table), but I changed my mind:
>  * the caller doesn't really need to disable softirqs across all
>    of the rhashtable_walk_* functions, only those parts that they
>    actually do within the lock need it
>  * maybe more importantly, it would still lead to massive lockdep
>    complaints - false positives, but hard to fix - because lockdep
>    wouldn't know about different ht->lock instances, and thus one
>    user of the code doing a walk w/o any locking (when it only ever
>    uses process context this is fine) vs. another user like in wifi
>    where we noticed this problem would still cause it to complain.
> 
> Cc: stable@...r.kernel.org
> Reported-by: Jouni Malinen <j@...fi>
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

Herbert and Johannes, I need some guidance.

It seems Herbert wants the softirq usage of rhashtables removed, but
since things have been like this for so long that's not the most
reasonable requirement if we can fix it more simply with Johannes's
patch especially for -stable.

Thanks.